Electric preload for variable effort steering system

ABSTRACT

A power steering system for a vehicle includes a hydraulic system for controlling resistance of a steering wheel. A valve of the hydraulic system is connected to a coil which varies positioning of the valve. A method for controlling the resistance of the steering wheel includes turning the steerable wheels of the vehicle in response to rotation of the steering wheel. The angle of the steering wheel and the speed of the vehicle are sensed and inputted into a controller. A lateral acceleration is determined based on the sensed signals by the controller. A recommended current rate of the coil is determined based on the lateral acceleration. An adjustment to the recommended current rate of the coil is determined based upon the speed of the vehicle. The recommended current rate is also modified based upon the adjustment. The modified current rate of the coil is then applied to the coil to vary the stiffness of the valve, which in turn varies the resistance to turning the steerable wheels of the vehicle.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional patent application of U.S. patentapplication Ser. No. 11/092,641, filed on Mar. 29, 2005, which claimsthe benefit of U.S. Provisional Patent Application Ser. No. 60/650,445for an ELECTRIC PRELOAD FOR VARIABLE EFFORT STEERING SYSTEM, filed onFeb. 4, 2005, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a power steering apparatus for a vehiclehaving an electromagnetic control apparatus of the variable reluctancetype for varying the driver steering effort required to produce a givenlevel of power assist.

BACKGROUND OF THE INVENTION

Power steering systems assist a drive in turning at least two wheels ofa vehicle. The driver engages a steering wheel and the power steeringsystem is operably disposed along the mechanical linkage between thesteering wheel and the turnable wheels of the vehicle. U.S. Pat. No.5,070,956 describes a hydraulic power assist steering system havingconventional relatively rotatable spool and valve body elements coupledto a vehicle steerable wheel and steering wheel for regulation of ahydraulic steering assist boost pressure, a torsion bar creating amechanical centering torque between the spool and valve body elements,and an integral electromagnetic mechanism which defines an additionalcoupling of variable resilience between the spool and valve bodyelements for adjusting driver steering effort required to produce agiven level of power assist.

SUMMARY OF THE INVENTION AND ADVANTAGES

The invention provides a method for controlling stiffness in a powersteering system for a vehicle. A pinion and a spool shaft are engagedwith respect to one another for turning at least two wheels of a vehiclein response to rotation of a steering wheel from an on-centerorientation. Movement of the pinion is variably assisted with ahydraulic power steering device having a valve movable between a closedconfiguration and an open configuration. The valve is moved from theopen configuration to the closed configuration in response to rotationof the spool shaft relative to the pinion. A torsion rod having a firststiffness is disposed between the pinion and spool shaft to resistrotation of the spool shaft relative to the pinion and resist movementof the valve from the open configuration to the closed configuration. Avariable magnetic coupling having a second stiffness is disposed inparallel with the torsion rod to resist rotation of the spool shaftrelative to the pinion. The first stiffness and the second stiffnesscooperate to define an overall valve stiffness. The second stiffnessgenerated by the variable magnetic coupling is varied with an electriccoil to vary the overall valve stiffness. The second stiffness generatedby the variable magnetic coupling is maximized when the steering wheelis in the on-center orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated asthe same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a longitudinal cross-sectional view of a motor vehicle powersteering gear according to the exemplary embodiment of the invention;

FIG. 2 is a graph illustrating valve stiffness generated by previouspower steering systems relative to an angle between a valve body andspool valve portion;

FIG. 3 is a second graph illustrating current drawn to generate amagnetic field with a magnetic coupling of previous power steeringsystems relative to an angle of a steering wheel;

FIG. 4 is a schematic drawing of a controller for controlling an amountof current drawn by a magnetic coupling according to the exemplaryembodiment of the invention;

FIG. 5 is a third graph illustrating valve stiffness generated by theexemplary power steering systems relative to an angle between a valvebody and spool valve portion; and

FIG. 6 is a fourth graph illustrating current drawn to generate amagnetic field with the magnetic coupling relative to an angle of asteering wheel according to the exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

U.S. Pat. Nos. 5,738,182; 5,119,898; 4,454,801; and 3,022,772 areincorporated by the reference to the present application. Theincorporated patents set forth the structure and operation of well-knownpower steering systems.

A variable effort motor vehicle power steering system 10 according tothe exemplary embodiment of the invention includes a housing 12. Apinion 14 having a plurality of gear teeth 16 is rotatably supported inhousing 12 by a roller bearing 18 and a bearing 20. A rack bar 22 havinga plurality of rack teeth meshing with the gear teeth 16 on the pinion14 is supported by the housing 12 for movement perpendicular to thepinion 14 in response to rotation of the pinion 14. The ends of rack bar22 (not shown) are connected to steerable wheels of the motor vehicle ina conventional manner.

A spool shaft 24 protrudes into the housing 12 through a cover 26 andannular fluid seal. Spool shaft 24 is supported on cover 26 by a bearing30 and is provided with an internal axial bore 32. A proportionalcontrol valve body 34 is supported for rotation about the spool shaft 24in the housing 12, similar to the valve described in U.S. Pat. Nos.4,454,801 and 3,022,772. A torsion rod 36 extends in bore 32 of spoolshaft 24. A serrated inboard end 38 of the torsion rod 36 protrudesbeyond a corresponding end 40 of the spool shaft 24 within the housing12 and is force fitted in a bore 42 defined by the pinion 14. Anoutboard end 44 of torsion rod 36 is connected to an outboard end 46 ofspool shaft 24 by a pin 48. Outboard end 46 of spool shaft 24 isconnected to a manual steering wheel, not shown, of the motor vehiclefor rotation in a conventional manner. A lost motion connection betweenthe spool shaft 24 and the pinion 14 allows for twist (e.g. about 7degrees) in the torsion rod 36 and relative angular movement between thepinion 14 and the spool shaft 24.

The valve body 34 surrounds a spool valve portion 50 of spool shaft 24within the housing 12 and is rotatably supported by a pair of sleeves 52and 54. The valve body 34 and spool valve portion 50 cooperate to definea valve 128. When the valve body 34 and spool valve portion 50 arealigned, or on-center with respect to one another, the valve 128 is openand hydraulic assistance is not provided to assist steering. As thevalve body 34 and spool valve portion 50 become increasingly angled withrespect to one another, the valve 128 incrementally closes and hydraulicassistance increases to assist steering. A pair of radial pins 56, 58fixes the valve body 34 for rotation with pinion 14. The valve body 34is also fixed for rotation with the inboard end 38 of torsion rod 36.Torsional flexure of torsion rod 36 thus produces relative rotationbetween the valve body 34 and the spool valve portion 50. This relativerotation opens and closes various orifices defined between the valvebody 34 and the spool valve portion 50 to regulate the pressure of fluidfrom a power steering pump (not shown) to a steering assist fluid motor(not shown). Details of the structure and operation of the fluidpressure regulation can be found in the prior art, including thepreviously mentioned and incorporated by reference U.S. Pat. Nos.5,738,182; 5,119,898; 4,454,801; and 3,022,772. However, this structureand operation is merely background and environment; its precise natureis not relevant to the understanding of this invention, except to notethat, the more the valve body 34 is rotated relative to the spool valveportion 50, in either direction, from on-center position wherein torsionrod 36 is unflexed or untwisted, the greater will be the differentialfluid pressure and consequent steering assist force in the correspondingdirection. The on-center position of the torsion rod 36 corresponds toon-center orientation of the steering wheel of the vehicle and alsocorresponds to the turnable wheels of the vehicle being substantiallyaligned with a longitudinal axis of the vehicle.

The torsion rod 36 defines a first valve stiffness of the steeringsystem 10. A magnetic coupling 60 defines a second valve stiffness ofthe steering system 10. Valve stiffness relates to the stiffnessassociated with rotating the valve body 34 and the spool valve portion50 relative to one another to engage hydraulic steering assist. Steeringstiffness relates to the overall stiffness of the steering system. Valvestiffness and steering stiffness are related in that, generally, thegreater the valve stiffness the greater the steering stiffness. However,steering stiffness corresponds to vehicle speed and lateral accelerationof the vehicle, while valve stiffness is not necessarily related tovehicle speed and lateral acceleration of the vehicle. The magneticcoupling 60 and the torsion rod 36 are operably disposed in parallel toone another, cooperating to define an overall valve stiffness of thesteering system 10. The second stiffness is variable, as set forth morefully below.

The magnetic coupling includes a first member 62 pressed onto sleeve 54such that the valve body 34, the first member 62, and the sleeves 52, 54are rotationally fixed relative to one another for concurrent rotationabout the spool shaft 24. The first member 62 includes a hub portion 64extending axially in the outboard direction and a tooth portion 66extending radially outward from hub portion 64, ending in a plurality(26 in this embodiment, although 24 may be preferred) of radiallyoutwardly projecting teeth 68.

The magnetic coupling 60 also includes a second member 70 isrotationally fixed by plastic injection 72 to the spool shaft 24. Thus,relative rotation between spool shaft 24 and pinion 14/valve body 34results in corresponding relative rotation between the first member 62and the second member 70. The second member 70 includes a non-magnetichub portion 74 and a magnetic tooth portion 76 having teeth 78. The hubportion 74 extends radially outward from the spool shaft 24 and thetooth portion 76 extends from the hub portion 74 axially toward and overthe teeth 68 of the first member 62. The tooth portions 66, 76 areradially spaced from another, the tooth portion 76 surround the toothportion 66. Each tooth 78 is directed radially inwardly toward acorresponding tooth 68. When the valve body 34 and steering wheel areon-center, the teeth 68, 78 are radially aligned.

An electric coil 80 receives a current at rate to generate a magneticfield tending to urge the teeth 68, 78 into alignment. The coil 80 iswound in an insulating bobbin 82 and retained in axial orientationrelative to the housing 12. The hub portion 64 of the first memberextends through the coil 80. The teeth 68 are disposed axially adjacentcoil 80. The housing 12 is made of a magnetic material such as malleablecast iron. The valve body 34 is made of a magnetic material such assteel. The first member 62 is made of magnetic phosphorus powdered ironin a powdered metal process, as is tooth portion 76 of second member 70.Hub portion 74 of second member 70 is made of a stiff, non-magneticmaterial such as stainless steel. Cover 26 is made of a non-magneticmaterial such as aluminum. A magnetic flux circuit is thus definedaround coil 80 as shown by the dashed line surrounding the coil 80,across a radially outer air gap 84 between housing 12 and tooth portion76 of second member 70, through tooth portion 76 and teeth 78, acrossthe air gap 86 between teeth 78 and opposing teeth 68 of the firstmember 62 and through hub portion 64 and valve body 34 back to thehousing 12 through an air gap. Since hub portion 74 of second member 70and cover 26 are non-magnetic, there is no significant leakage fluxbypassing the air gaps and this concentrates the maximum flux generatedby current in coil 80 across these air gaps. In addition, since teeth 68and 78 are radially rather than axially disposed relative to each other,magnetic forces between the valve body 34 and the spool shaft 24 areradial and circumferential, minimizing axial loads between the valvebody 34 and the spool shaft 24. The rate of current directed through thecoil 80 can be changed to change the strength of the magnetic field and,thus, the torque urging the teeth 68, 78 into alignment.

The magnetic coupling 60 operates as a variable reluctance torquegenerator. There are no permanent magnets and the only magnetic flux isthat generated by an electric current provided through coil 80. Thisflux is concentrated in the magnetic material around the magneticcircuit described above, with low fringing and leakage flux and withthree significant air gaps in series. The radially outer air gap 84between housing 12 and tooth portion 76 of second member 70 does notvary significantly with relative rotation of the teeth. The air gap 86between opposing teeth 68 and 78 comprises the plurality of parallel airgaps between all opposing teeth. This varies with relative rotationbetween the first and second members 62, 70. In the centered position ofspool shaft 24 and torsion rod 36, teeth 68 of first member 62 arealigned with teeth 78 of the second member 70 and the air gaps 86 areminimized. A current in coil 80 generates a torque between first member62 and second member 70 which attempts to reduce the total reluctance,minimizing the air gaps 86.

Without consideration of the torsion rode 36 and the magnetic coupling60, the stiffness of the steering system 10 is generally lowest when thetorsion bar 36 and steering wheel are on-center. As vehicle speedincreases and as the angle of the steering wheel from on-centerincreases, steering stiffness increases. It can be desirable to arrangethe steering system so that steering system stiffness is generally thesame throughout the range of steering wheel angle and throughout therange of vehicle speed. Since the stiffness is generally relativelysmaller when the steering wheel is on-center and generally relativelygreater when the steering wheel is off-center, it may be desirable topre-stiffen the steering valve to increase valve stiffness (tending toalign the valve body 34 and spool valve portion 50 and tending toprevent hydraulic steering assist) when the steering wheel is on-centerand reduce the pre-stiffness as the steering wheel moves from on-center,since the other components of the steering system will increase thestiffness of the system. In the exemplary embodiment of the invention,the magnetic coupling 60 is controlled by a controller 88 to maximizethe torque between the first and second members 62, 70 when the torsionrod 36 and steering wheel is on center and reduce the torque between thefirst and second members 62, 70 as the torsion rod 36 and steering wheelmove from on-center. This has been done in the past with mechanicaldevices.

FIG. 2 is a graph illustrating a previous relationship between valvestiffness (torque) and the angle of the valve body 34 and the spoolvalve portion 50 relative to one another. Line portions 90 and 92represent relatively small increases in torque as the valve body 34 andthe spool valve portion 50 rotate relative to one another fromon-center, represented by the Y-axis. Line portions 94 and 96 representrelatively large increases in torque as the valve body 34 and the spoolvalve portion 50 rotate relative to one another further from on-center.Line portions 98 and 100 represent relatively small increases in torqueas the valve body 34 and the spool valve portion 50 rotate relative toone another further from on-center. The line portions 90, 92, 98, 100are substantially parallel with one another. FIG. 3 is a second graphillustrating current drawn to generate a magnetic field with a magneticcoupling of the previous power steering systems versus to an angle ofthe valve body 34 and the spool valve portion 50 relative to oneanother. FIGS. 2 and 3 correspond with one another. The angle of thesteering wheel from on-center corresponds to the angle of the valve body34 and the spool valve portion 50 relative to one another. Line portions94 and 96 correspond to portions of line 102 where current is beingdrawn. The line portions 90, 92, 98, 100 correspond to portions of theline 102 where current is not being drawn. As shown in FIG. 3, nocurrent is being drawn when the valve body 34 and the spool valveportion 50 are on-center in the previous system.

FIG. 4 is a schematic drawing of the controller 88 (in dash line) forcontrolling the rate of current directed through the coil 80. Thecontroller 88 communicates with a vehicle speed sensor 110 and asteering wheel angle sensor 112. The speed sensor 110 communicates asignal corresponding to vehicle speed to a speed table 114 of thecontroller 88. The speed table 114 identifies a recommended current rateof the coil 80 in view of the signal received from the sensor 110, therecommendation is based on the performance of a calculation or based ona comparison of the signal to a table. For example, based on the signalfrom the sensor 110, the speed table 114 may communicate a recommendedcurrent of two amps to a summation operator 116 of the controller 88.

The angle sensor 112 communicates a signal corresponding to steeringwheel angle to a lateral acceleration calculator 120. The lateralacceleration calculator 120 also receives a signal from the speed sensor110. Data corresponding to speed and wheel angle are applied by thelateral acceleration calculator 120 to calculate the lateralacceleration, which is communicated to a lateral acceleration table 122.The lateral acceleration table 122 identifies a recommended current rateof the coil 80 in view of the signal received from the sensors 110 and112. The recommendation is based on the performance of a calculation orbased on the comparison of the signals to a table.

The speed sensor 110 also communicates a signal to a lateralacceleration modifier 118. The lateral acceleration modifier 118identifies a recommended adjustment to the current recommendation madeby the lateral acceleration table 122. The recommend adjustment is basedon the performance of a calculation or based on the comparison of thesignal to a table.

The recommendation from the lateral acceleration table 122 and therecommended adjustment made by the lateral acceleration modifier 118 arecommunicated to the multiplier operator 124. The multiplier operator 124modifies the recommendation of the lateral acceleration table 122 inview of the recommended adjustment made by the lateral accelerationmodifier 118. In operation, the recommendation of the lateralacceleration table 122 will be decreased by the multiplier operator 124if the speed is relatively low and will be increased by the multiplieroperator 124 if the speed is relatively high. The multiplier operator124 submits a modified current recommendation to the summation operator116 of the controller 88.

The following examples illustrate the operation of the exemplarycontroller 88. When the vehicle is not moving, the speed table 114 mayrecommend a current of zero amp. Also, when the vehicle is not moving,the lateral acceleration table 122 may recommend a current of threeamps. The lateral acceleration modifier 118 may recommend adjustment ofthe three amp recommendation to zero amp since speed is zero. In otherwords, the operational effect of the lateral acceleration modifier 118may be to recommend zero percent of the lateral acceleration table 122be accepted since speed is zero. The summation operator 116 willtherefore receive recommendations of zero amps from the speed table 114and zero amps from the multiplier operator 124. Therefore, the coil 80will not receive current in this example.

When the vehicle is moving at a speed of ten miles per hour, the speedtable 114 may recommend a current to the coil 80 of one amp. The lateralacceleration table 122 may recommend a current of three amps. Thelateral acceleration modifier 118 may recommend adjustment of the threeamp recommendation to thirty percent, or one amp (one third of the threeamp recommendation). The summation operator 116 will therefore receiverecommendations of one amp from the speed table 114 and one amp from themultiplier operator 124. Therefore, the coil 80 will receive two amps ofcurrent in this example.

When the vehicle is moving at a speed of thirty miles per hour, thespeed table 114 may recommend a current to the coil 80 of one amp. Thelateral acceleration table 122 may recommend a current of three amps.The lateral acceleration modifier 118 may recommend zero adjustment ofthe three amp recommendation, or full adoption of the recommendation ofthe lateral acceleration table 122. The summation operator 116 willtherefore receive recommendations of one amp from the speed table 114and three amps from the multiplier operator 124. Since the sum of thecurrent recommendations is greater than three, a current limitingoperator 130 of the exemplary controller 88 will limit therecommendation to three amps. Therefore, the coil 80 will receive threeamps of current in this example. The controller 88 will control acurrent generating device 126 to direct a current of three amps to thecoil 80.

FIG. 5 is a third graph illustrating the relationship between valvestiffness (torque) and the angle of the valve body 34 and the spoolvalve portion 50 relative to one another in the present invention. Lineportions 104 and 106 represent relatively large increases in torque asthe valve body 34 and the spool valve portion 50 rotate relative to oneanother from on-center. Line portions 108 and 110 represent relativelysmall increases in torque as the valve body 34 and the spool valveportion 50 rotate relative to one another further from on-center. Theline portions 108 and 110 are comparable to the line portions 90, 92,98, 100 in FIG. 2. FIG. 6 is a fourth graph illustrating current drawnto generate a magnetic field with a magnetic coupling 60 of the powersteering system 10 relative to an angle of a steering wheel. FIGS. 5 and6 correspond with one another. Line portions 104 and 106 correspond tothe portion of line 108 where current is being drawn. The line portions108 and 110 correspond to portions of the line 108 where current is notbeing drawn. As shown in FIG. 6, a maximum current is being drawn whenthe steering wheel is on-center.

The position of the steering wheel with respect to being on-center oroff-center corresponds to the positions of the valve body 34 and thespool valve portion 50 relative to one another. In other words, when thesteering wheel is on-center, the valve body 34 and the spool valveportion 50 are on-center relative to one another (the valve 128 beingopen, no hydraulic assist). Thus, FIG. 6 also implicitly shows that thecurrent is maximized when the valve body 34 and the spool valve portion50 are on-center relative to one another. The difference betweensteering wheel movement relative to on-center and valve body 34-spoolvalve portion 50 movement relative to on-center is that extent ofmovement. For example, the steering wheel can move sixty degreesoff-center while the valve body 34 and spool valve portion 50 cancorrespondingly move two degrees from on-center.

In operation, the pinion 14 and the spool shaft 24 are engaged with oneanother for turning at least two wheels of a vehicle in response torotation of a steering wheel from an on-center orientation. The pinion14 is variably assisted in movement with a hydraulic power steeringdevice having a valve body 34 movable between a closed configuration andan open configuration. The valve body 34 is moved from the openconfiguration to the closed configuration in response to rotation of thespool shaft 24 relative to the pinion 14 to assist the pinion 14 inrotation for turning the at least two wheels. The amount of assistanceprovided by the hydraulic power steering device is responsive to theextent that the valve body 34 and the spool valve portion 50 areradially offset from one another; the more radially offset, the moreassistance is provided. The details of the cooperation between the valvebody 34 and the spool valve portion 50 are set forth fully in thepatents incorporated by reference to this application. A torsion rod 36having a first stiffness is disposed between the pinion 14 and spoolshaft 24 to resist rotation of the spool shaft 24 relative to the pinion14 and resist movement of the valve body 34 from the open configurationto the closed configuration. A variable magnetic coupling 60 having asecond stiffness is disposed in parallel with the torsion rod 36 toresist rotation of the spool shaft 24 relative to the pinion 14 andresist movement of the valve body 34 from the closed configuration tothe open configuration. The first stiffness and second stiffnesscooperate to define an overall steering stiffness. The second stiffnessgenerated by the variable magnetic coupling 60 is varied with anelectric coil 80 to vary the overall steering stiffness. The secondstiffness is maximized when the steering wheel is in the on-centerorientation.

The relationship between current and changes in the angle between thevalve body 34 and the spool valve portion 50 is determined through asubjective evaluation of the vehicle. The relationship is usually veryspecific to any particular operating environment. For example, if aconstant, direct feel of the vehicle is desired, the knee of the valvestiffness curve will be sharper, or more pronounced. On the other hand,if a valley feel is desired, the knee of the valve stiffness curve willbe more subtle and positioned at a point that corresponds to thestiffness of the entire system (tires, column, etc) beginning to becomestiff.

While the invention has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method for controlling a resistance to turning steerable wheels ofa vehicle including a controller, a steering wheel angle sensorresponsive to rotation of a steering wheel, a speed sensor, and a coilfor varying stiffness of a valve, said method comprising the steps of:turning the wheels of the vehicle in response to rotation of thesteering wheel; sensing a signal corresponding to an angle of thesteering wheel; sensing a signal corresponding to a speed of thevehicle; inputting the sensed signals for the angle of the steeringwheel and the speed of the vehicle into the controller; determining alateral acceleration based on the sensed signals; determining arecommended current rate of the coil based upon the lateralacceleration; determining an adjustment to the recommended current rateof the coil based upon the sensed signal for the speed of the vehicle;multiplying the recommended current rate of the coil and the adjustmentto the recommended current rate of the coil to produce a modifiedcurrent rate of the coil; and applying the modified current rate of thecoil to energize the coil and vary the stiffness of the valve to varythe resistance to turning the steerable wheels.
 2. A method as set forthin claim 1 further comprising the steps of: determining a secondrecommended current rate of the coil based upon the sensed speed of thevehicle; adjusting the modified current rate of the coil based upon thesecond recommended current rate of the coil to determine an adjustedmodified current rate of the coil; and said step of applying themodified current rate of the coil is further defined as the step ofapplying the adjusted modified current rate of the coil to energize thecoil and vary the stiffness of the valve to vary the resistance toturning the steerable wheels.
 3. A method as set forth in claim 2wherein said step of adjusting the modified current rate of the coil isfurther defined as the step of performing a mathematical function to themodified current rate of the coil and the second recommended currentrate of the coil to determine the adjusted modified current rate of thecoil.
 4. A method as set forth in claim 3 wherein said step ofperforming the mathematical function is further defined as summing themodified current rate of the coil and the second recommended currentrate of the coil to determine the adjusted modified current rate of thecoil.
 5. A method as set forth in claim 1 further comprising the step oflimiting the modified current rate of the coil applied to energize thecoil to a current range of the coil.
 6. A method as set forth in claim 5wherein said step of limiting the modified current rate of the coil isfurther defined as the step of limiting the modified current rate of thecoil applied to energize the coil to no more than 3.0 Amps if themodified current rate of the coil is outside of the current range of thecoil.
 7. A method as set forth in claim 1 wherein said step ofdetermining the recommended current rate of the coil based upon thelateral acceleration is further defined as the step of calculating therecommended current rate of the coil based upon the lateralacceleration.
 8. A method as set forth in claim 1 wherein said step ofdetermining the recommended current rate of the coil based upon thelateral acceleration is further defined as the step of comparing thelateral acceleration to a table to determine the recommended currentrate of the coil.
 9. A method as set forth in claim 1 wherein said stepof determining the adjustment to the recommended current rate of thecoil based upon the sensed signals is further defined as the step ofcalculating the recommended current rate of the coil based upon thesensed signals to determine the adjustment to the recommended currentrate of the coil.
 10. A method as set forth in claim 1 wherein said stepof determining the adjustment to the recommended current rate of thecoil based upon the sensed signals is further defined as the step ofcomparing the sensed signals to a table to determine the adjustment tothe recommended current rate of the coil.
 11. A method for controlling aresistance to turning steerable wheels of a vehicle including acontroller, a steering wheel angle sensor responsive to rotation of asteering wheel, a speed sensor, and a coil for varying stiffness of avalve, said method comprising the steps of: turning the wheels of thevehicle in response to rotation of the steering wheel; sensing a signalcorresponding to an angle of the steering wheel; sensing a signalcorresponding to a speed of the vehicle; inputting the sensed signalsfor the angle of the steering wheel and the speed of the vehicle intothe controller; determining a lateral acceleration based on the sensedsignals; determining a recommended current rate of the coil based uponthe lateral acceleration; determining an adjustment to the recommendedcurrent rate of the coil based upon the sensed signal for the speed ofthe vehicle; modifying the recommended current rate of the coil bydecreasing the recommended current rate of the coil if the vehicle speedis below a first predetermined speed to determine a modified currentrate of the coil and increasing the recommended current rate of the coilif the vehicle speed is above a second predetermined speed to determinethe modified current rate of the coil; and applying the modified currentrate of the coil to energize the coil and vary the stiffness of thevalve to vary the resistance to turning the steerable wheels.
 12. Amethod as set forth in claim 11 further comprising the steps of:determining a second recommended current rate of the coil based upon thesensed speed of the vehicle; adjusting the modified current rate of thecoil based upon the second recommended current rate of the coil todetermine an adjusted modified current rate of the coil; and said stepof applying the modified current rate of the coil is further defined asthe step of applying the adjusted modified current rate of the coil toenergize the coil and vary the stiffness of the valve to vary theresistance to turning the steerable wheels.
 13. A method as set forth inclaim 12 wherein said step of adjusting the modified current rate of thecoil is further defined as the step of performing a mathematicalfunction to the modified current rate of the coil and the secondrecommended current rate of the coil to determine the adjusted modifiedcurrent rate of the coil.
 14. A method as set forth in claim 13 whereinsaid step of performing the mathematical function is further defined assumming the modified current rate of the coil and the second recommendedcurrent rate of the coil to determine the adjusted modified current rateof the coil.